Glass bottles or flasks are used in a variety of applications. For example, Erlenmeyer flasks are used in laboratory environments for growing or cultivating a wide range of things including orchids, recombinant DNA, tissue cultures and general laboratory use. In many applications it is necessary to stopper these bottles and flasks in order to establish a good fluid seal against leakage. Rubber stoppers and corks are historically used, in part because they easily adjust to the wide I.D. and O.D. tolerances of glass receptacles. But conventional corks and stoppers are not wholly suitable because they become contaminated and cannot be easily sterilized. Often laboratory personnel simply use plastic film, tin foil or the like with a rubber band to cover the mouth of the bottle or flask. These are not truly dependable and while they do keep out contaminants to some degree, they do not function well to prevent spillage if the flask is tilted or falls over. Frequently a piece of fabric is stuffed into the opening in the bottle to act as a plug or stopper. A fabric plug suffers from the same shortcomings as enumerated above, although it does address another problem: it allows the bottle to breathe. That is, air and oxygen can be exchanged through the fabric plug and gas pressure buildup can be released through the fabric too. These are important considerations when aerobic activity is occurring in the flask. A further problem is that most of the stoppers or plugs do not protect the mouth of the bottle against damage: if the flask should fall over, the lip of the flask hits the surface and cracking or shattering is likely. One attempt to provide an improved cover uses a very soft plastic which is difficult to install and remove and is extremely expensive. In addition it uses a sponge-like insert in the cover to permit a measure of breathing, but the spongy material is susceptible to contamination and is difficult to sterilize.